JP4284927B2

JP4284927B2 - Aperture limiting element and optical head device

Info

Publication number: JP4284927B2
Application number: JP2002166911A
Authority: JP
Inventors: 真弘村川; 公貴梨子; 好晴大井
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2002-06-07
Filing date: 2002-06-07
Publication date: 2009-06-24
Anticipated expiration: 2022-06-07
Also published as: JP2004014034A

Description

【０００１】
【発明の属する技術分野】
本発明は、開口制限素子および光ヘッド装置に関し、特に光ヘッド装置に関しては光記録媒体の情報の記録および再生を行う光ヘッド装置に関する。
【０００２】
【従来の技術】
同一の光ヘッド装置を用いて、規格の異なる光ディスクであるＣＤまたはＤＶＤの記録および再生を行う、ＣＤ／ＤＶＤ互換型光ヘッド装置が製品化されている。ＣＤとＤＶＤのそれぞれの光ディスクでは、記録および再生に用いる光の波長帯、光ディスクの厚さ、記録密度などの規格が異なるため、ＣＤの記録および再生時とＤＶＤの記録および再生時とでそれぞれ光学系の開口数を変える必要がある。
【０００３】
【発明が解決しようとする課題】
波長６５０ｎｍのＤＶＤ用に設計された開口数０．６程度の対物レンズを用いて、ＣＤの記録および再生を実行するために、図１１に示す開口制限素子５０１を用いて、ＣＤ用の波長７８０ｎｍに対し、開口数を切り替える方法が提案されている。開口制限素子５０１は、透明基板５１上に位相差調整用の誘電体多層膜５２が形成された中央領域Ｃと波長選択性の誘電体多層膜５３が形成された周辺領域Ｄを有している。また、５４は反射防止膜である。
【０００４】
波長選択性の誘電体多層膜５３は、ＤＶＤ用の光束を透過させるが、ＣＤ用の光束は透過させない機能を有する。一方、位相差調整用の誘電体多層膜５２は、ＤＶＤ用の光束とＣＤ用の光束をともに透過させるとともに、中央領域Ｃと周辺領域ＤにおけるＤＶＤ用の光束の位相を揃える機能を有している。
【０００５】
しかしながら、開口制限素子５０１は、中央領域Ｃと周辺領域Ｄとを形成する製造工程において、２０層前後の誘電体多層膜の形成を２回、リフトオフ用のマスクの形成を２回、リフトオフを２回繰り返すなどの複雑な工程が必要となる。この複雑な工程が原因で歩留まりの向上が妨げられ、簡略化された工程により作製された開口制限素子が望まれていた。
本発明は、製造工程数が少なく容易に作製でき、かつ生産歩留まりのよい開口制限素子、およびそれを用いた光ヘッド装置を提供することを目的とする。
【０００６】
【課題を解決するための手段】
本発明は、入射する異なる波長λ_１と波長λ_２（λ_１＜λ_２）の２つの光束のうち、いずれか一方の光束のみを透過するフィルタ域と両方の光束を透過する透明域とを有する開口制限素子であって、前記開口制限素子は２枚の透明基板を備え、２枚の前記透明基板間における周辺領域には平滑な表面を有する波長選択性の誘電体多層膜からなる前記フィルタ域が形成され、前記周辺領域により囲まれる中央領域において、一方の前記透明基板の面のうち他方の前記透明基板と対向する側に、前記周辺領域に対して一定の段差を有する凹状部が形成されるとともに透明媒質からなる前記透明域が形成されており、かつ前記誘電体多層膜の膜構成と前記透明媒質の屈折率とは、前記フィルタ域および前記透明域を透過する光束に対して前記フィルタ域と前記透明域とで光束の位相が揃うように調整されていることを特徴とする開口制限素子を提供する。
【０００７】
また、前記誘電体多層膜の総膜厚および前記波長λ _１の光に対する平均屈折率をそれぞれｄ _ｆ２、ｎ _ｆ２、前記透明媒質の屈折率をｎ _ｓ２、前記凹状部の段差をｄ _ａ、前記波長λ _１の光に対する前記透明基板の屈折率をｎ _ａとするとき、ｎ _ｆ２・ｄ _ｆ２＋ｎ _ａ・ｄ _ａ＝ｎ _ｓ２・（ｄ _ｆ２＋ｄ _ａ）の関係を満たす上記の開口制限素子を提供する。また、前記透明基板間には、前記誘電体多層膜と前記透明媒質とに加えさらに有機物薄膜からなる波長板および／または回折格子が積層されている上記の開口制限素子を提供する。
【０００８】
また、異なる波長λ_１と波長λ_２（λ_１＜λ_２）の２つの光束をそれぞれ出射する２つの光源と、２つの光束を光記録媒体に集光するための対物レンズと、集光して光記録媒体により反射した２つの光束を検出する光検出器とを備える光ヘッド装置であって、２つの光源と対物レンズとの間において２つの光束の共通光路中に上記の開口制限素子が配設されている光ヘッド装置を提供する。さらに、入射する異なる波長λ_１と波長λ_２（λ_１＜λ_２）の２つの光束のうち、いずれか一方の光束のみを透過するフィルタ域と両方の光束を透過する屈折率ｎ_ｓ２の透明媒質を含む透明域とを有する開口制限素子の製造方法であって、前記開口制限素子は、前記波長λ _１の光に対する屈折率がｎ _ａとなる透明基板の平面上に総膜厚ｄ_ｆ２で前記波長λ_１の光に対する平均屈折率ｎ_ｆ２となる光学多層膜を形成し、光学多層膜上にフォトレジストを積層し、透明域となる中央領域のみフォトリソグラフィおよびエッチングによって光学多層膜を食刻するとともに透明基板をｎ_ｆ２・ｄ_ｆ２＋ｎ_ａ・ｄ_ａ＝ｎ_ｓ２・（ｄ_ｆ２＋ｄ_ａ）の関係を満たす段差ｄ_ａまで食刻し、フィルタ域となる周辺領域のレジストを除去し、前記中央領域に前記透明媒質を充填してもう一方の透明基板と接着固定し、２枚の前記透明基板の対向しない側の表面が平滑となる開口制限素子の製造方法を提供する。
【０００９】
【発明の実施の形態】
図２に本発明の開口制限素子１０１を搭載した光ヘッド装置の構成例を示す。図２に示す光ヘッド装置は、ＤＶＤ６およびＣＤ７の情報の記録および再生を行う光ヘッド装置であり、光源としてＤＶＤ用の波長λ_１の光束Ｌ_１を出射する半導体レーザ１ＡとＣＤ用の波長λ_２の光束Ｌ_２を出射する半導体レーザ１Ｂとを備えている。
【００１０】
半導体レーザ１Ａおよび１Ｂを出射した光束Ｌ_１、Ｌ_２はそれぞれ、ビームスプリッタ２および３で反射され、コリメートレンズ４を透過後、開口制限素子１０１を透過して、対物レンズ５によって、ＤＶＤ６またはＣＤ７上に集光される。ＤＶＤ６またはＣＤ７で反射した光は、上記の進行方向とは逆に進みビームスプリッタ２および３を透過して、最終的に光検出器８上に集光する。また、９は位相板であり、４分の１波長板を用いることで、ＣＤ７への情報記録時に半導体レーザ１Ｂの出射光と同一偏光方向の戻り光が、半導体レーザ１Ｂに進入するのを防ぎ、半導体レーザ１Ｂの発振特性を安定させることができる。
【００１１】
図１に本発明の第１実施態様として開口制限素子１０１の（ａ）断面図と、（ｂ）平面図を示す。開口制限素子１０１は、透明基板１１の下面の周辺領域Ｂに、厚さがｄ_ｆ１で波長選択性の誘電体多層膜１３が形成されており、透明媒質である接着剤１４によって、透明基板１１と透明基板１２が固定されている。このとき、中央領域Ａには接着剤１４が充填されており、開口制限素子１０１を透過する光束Ｌ１のうち、中央領域Ａを透過する部分と周辺領域Ｂを透過する部分とで位相が揃うように、誘電体多層膜１３の膜構成および接着剤１４の屈折率を調整する。
【００１２】
すなわち、波長選択性の誘電体多層膜１３の平均屈折率ｎ_ｆ１と、接着剤１４の屈折率ｎ_ｓ１が等しくなるようにする。ここで、平均屈折率とは、多層膜を構成する各層の屈折率と厚さとの積の総和を、多層膜の総厚（各層厚の合計）で除算したものを意味する。
【００１３】
また、開口制限素子１０１は、図１の（ｂ）に示すように素子表面での反射を防ぐため、透明基板１１の一方の面に反射防止膜１５、透明基板１２の他方の面に反射防止膜１６がそれぞれ施されていることが好ましい。透明基板１１および１２としては、ガラス、石英ガラスなどの光学的に等方的な材料を用いることができる。
【００１４】
波長選択性の誘電体多層膜１３としては、ＤＶＤ用の波長λ_１の光束Ｌ_１に対し９０％以上の透過率、ＣＤ用の波長λ_２の光束Ｌ_２に対し２０％以下の透過率を有する層数が２０層前後の誘電体多層膜を使用できる。ただし、ＣＤ用の光束Ｌ_２の透過率としては、ＣＤの情報記録面上への不要な光の集光を防ぐため、１０％以下が好ましい。したがって、波長選択性の誘電体多層膜１３を周辺領域Ｂに成膜することで、周辺領域Ｂはフィルタ域、中央領域Ａは透明域になり、開口制限素子１０１はＣＤ用の光束Ｌ_２の径を中央領域Ａの径に制限できる。
【００１５】
また、反射防止膜１５および１６としては、ＤＶＤ用の光束Ｌ_１とＣＤ用の光束Ｌ_２が、ともに９８％以上透過し、容易に作製できる４から５層程度の誘電体多層膜を使用できる。
【００１６】
本発明の開口制限素子１０１を搭載した図２の光ヘッド装置では、ＤＶＤ用の光束Ｌ_１は、開口制限素子１０１によって、径を制限されることなく、対物レンズ５を用いて開口数０．６の光束Ｌ_１としてＤＶＤ６上に集光される。一方、ＣＤ用の光束Ｌ_２は、開口制限素子１０１によって、径を制限され、対物レンズ５を用いて開口数０．４５から０．５程度までの光束Ｌ_２としてＣＤ７上に集光される。すなわち、ＤＶＤ６やＣＤ７のように厚さの異なる光ディスクでも同一の対物レンズを用いて、おのおのの規格に合った開口数で光ディスク上に集光できるので、記録および再生を安定に実現できる。
【００１７】
次に、図３を用いて、本発明の開口制限素子１０１の製造方法について説明する。まず、図３の（ａ）に示すように、透明基板１１の一方の面に反射防止膜１５を、他方の面に厚さがｄ_ｆ１の波長選択性の誘電体多層膜１３を、真空蒸着法またはスパッタリング法にて成膜する。そののち、図３の（ｂ）に示すように誘電体多層膜１３上にフォトレジスト２２を積層し、フォトリソグラフィの技術を用いてパターニングして、図３の（ｃ）に示すように、誘電体多層膜１３をドライエッチング法にて食刻する。
【００１８】
さらに、残ったフォトレジスト２２を、剥離した後、接着剤１４を用いて、反射防止膜１６が成膜された透明基板１２を接着して、図３の（ｄ）に示すように開口制限素子１０１が作製される。
【００１９】
図４に本発明の第２実施態様として開口制限素子２０１の断面図を示す。開口制限素子２０１は、透明基板１７の一方の面に、波長選択性の誘電体多層膜１８が形成された周辺領域と、凹状に食刻された中央領域からなり、透明媒質である接着剤１９によって、透明基板１７と透明基板１２とが固定されている。第１実施態様の開口制限素子１０１と異なる点は、透明基板１７の一方の面も中央領域が一部食刻されたことである。
【００２０】
このとき、中央領域には接着剤１９が充填されており、開口制限素子２０１を透過する光束Ｌ１のうち、中央領域を透過する部分と周辺領域を透過する部分とで位相が揃うように、誘電体多層膜１８の膜構成および接着剤１９の屈折率を調整する。すなわち、誘電体多層１８の総膜厚ｄ_ｆ２、平均屈折率ｎ_ｆ２、接着剤１９の屈折率ｎ_ｓ２、透明基板１７の凹状に食刻された部分の段差（深さ）ｄ_ａ、屈折率ｎ_ａを用いて、ｎ_ｆ２ｄ_ｆ２＋ｎ_ａｄ_ａ＝ｎ_ｓ２（ｄ_ｆ２＋ｄ_ａ）の関係を満たすようにする。
【００２１】
また、開口制限素子２０１は、素子表面での反射を防ぐため、図４に示すように透明基板１７の他方の面に反射防止膜１５、透明基板１２の一方の面に反射防止膜１６がそれぞれ施されていてもよい。透明基板１７および１２としては、ガラス、石英ガラスなどの光学的に等方的な材料を用いることができる。
【００２２】
波長選択性の誘電体多層膜１８としては、ＤＶＤ用の波長λ_１の光束Ｌ_１に対し９０％以上の透過率、ＣＤ用の波長λ_２の光束Ｌ_２に対し２０％以下の透過率を有する層数が２０層前後の誘電体多層膜を使用できる。ただし、ＣＤ用の光束Ｌ_２の透過率としては、ＣＤの情報記録面上への不要な光の集光を防ぐため、１０％以下が好ましい。したがって、波長選択性の誘電体多層膜１８を周辺領域に成膜することで、周辺領域Ｂはフィルタ域、中央領域は透明域になり、開口制限素子２０１はＣＤ用の光束Ｌ_２の径を中央領域Ａの径に制限できる。
【００２３】
図２の光ヘッド装置において、第１実施態様の開口制限素子１０１のかわりに第２実施態様の開口制限素子２０１を用いても、同様に、ＤＶＤ６やＣＤ７のように厚さの異なる光ディスクに対して、記録および再生を安定に実現できる。
【００２４】
本発明の開口制限素子１０１および２０１においては、その構成上層数が２０層前後の誘電体多層膜の成膜工程が１つ、ドライエッチング工程またはリフトオフ工程が１つといった簡単な工程で作製でき、生産性が高くかつ歩留まりよく開口制限素子を作製できる。
【００２５】
次に、図５を用いて、本発明の開口制限素子２０１の製造方法について説明する。まず、図５の（ａ）に示すように、透明基板１７の一方の面に反射防止膜１５を、他方の面に厚さがｄ_ｆ２の波長選択性の誘電体多層膜１８を、真空蒸着法またはスパッタリング法にて成膜する。そののち、図５の（ｂ）に示すように誘電体多層膜１８上にフォトレジスト２２を積層し、フォトリソグラフィの技術を用いてパターニングして、図５の（ｃ）に示すように、誘電体多層膜１８、および透明基板１７をドライエッチング法にて食刻する。
【００２６】
さらに、残ったフォトレジスト２２を剥離した後、接着剤１９を用いて、反射防止膜１６が成膜された透明基板１２を接着して、図５の（ｄ）に示す開口制限素子２０１が作製される。
【００２７】
図６は、本発明の第３実施態様としての開口制限素子１０２の断面図である。開口制限素子１０２は、図１に示す第１実施態様の開口制限素子１０１における接着剤１４と透明基板１２との界面に、位相板や回折格子などの光の状態を制御する光学素子２３を挿入した構成である。図６の他の符号で図１と同じものは、同じ要素を示す。第３実施態様の開口制限素子１０２を、光ヘッド装置に搭載する場合、光ヘッド装置を構成する素子の数を減らすこともでき、光ヘッド装置の小型化が実現できて好ましい。
【００２８】
図７は、本発明の第４実施態様としての開口制限素子２０２の断面図である。開口制限素子２０２は、図３に示す第２実施態様の開口制限素子２０１における接着剤１９と透明基板１２との界面に、位相板や回折格子などの光の状態を制御する光学素子２３を挿入した構成である。図７の他の符号で図３と同じものは、同じ要素を示す。第４実施態様の開口制限素子２０２を、光ヘッド装置に搭載する場合、光ヘッド装置を構成する素子の数を減らすこともでき、光ヘッド装置の小型化が実現できて好ましい。
【００２９】
図８は、本発明の第５実施態様の開口制限素子１０３を搭載した光ヘッド装置を示す概念的断面図であり、ＤＶＤ６およびＣＤ７の情報の記録および再生を行う光ヘッド装置である。４１Ａは、ＤＶＤ用の半導体レーザ１Ａと光検出器８Ａを集積したユニットであり、４１Ｂは、ＣＤ用の半導体レーザ１Ｂと光検出器８Ｂ、偏光依存性のないホログラム素子１０を集積し一体化したホログラムユニットである。光ヘッド装置にユニットを用いると、光ヘッド装置を小型化できる。なお、図１０中の符号で説明のない図１中と同符号のものは、図１中と同じ要素を意味する。
【００３０】
開口制限素子１０３は、例えば図９に示すように、図１の第１実施態様の開口制限素子１０１における接着剤１４と透明基板１２との界面に、位相板２０と偏光ホログラム格子２１を、挿入した構成である（第５実施態様）。図９の他の符号で図１と同じものは、同じ要素を示す。また、開口制限素子１０３の替わりに、図１０に示す開口制限素子２０３を用いてもよい（第６実施態様）。開口制限素子２０３は、図４の第２実施態様の開口制限素子２０１における接着剤１９と透明基板１２との界面に、位相板２０と偏光ホログラム格子２１を、挿入した構成である。図１０の他の符号で図４と同じものは、同じ要素を示す。
【００３１】
図９に示された開口制限素子１０３における位相板２０により発生するリタデーション値を、ＤＶＤ用の光束Ｌ_１に対し４分の５波長、さらにＣＤ用の光束Ｌ_２に対し１波長になるように選択する。この選択によって、光束Ｌ_１は偏光ホログラム格子２１の機能により、往路では開口制限素子１０３にて回折されずに直進透過するが、ＤＶＤ６反射後の復路では開口制限素子１０３にて回折され、光検出器８Ａに集光される（図８参照）。一方、光束Ｌ_２は往路、復路ともに偏光ホログラム格子２１では回折せずに開口制限素子１０３を透過できる。
【００３２】
偏光ホログラム格子２１としては、例えば高分子液晶に、フォトリソグラフィとエッチングの技術を用いて、断面形状が凹凸型のホログラム格子を加工した後、高分子液晶の常光屈折率または異常光屈折率に等しい光学的に等方的な材料をホログラム格子の例えば凹部に充填したものを用いることができる。
【００３３】
図９に示した本発明の第３実施態様の開口制限素子１０３を搭載した光ヘッド装置（図８）では、開口制限素子１０３を用いることにより、図２に示した開口制限素子１０１を搭載した光ヘッド装置と同様に、単一の対物レンズを用いてＣＤおよびＤＶＤの記録および再生を安定に実現できる。さらに、開口制限素子１０３は、偏光ホログラム格子２１と位相板２０を積層することで、開口制限機能、ビームスプリッタ機能および偏光制御機能をひとつの光学素子に持たせており、図２に示した光ヘッド装置に比べ、小型の光ヘッド装置を実現できる。
【００３４】
【実施例】
本実施例は、ＤＶＤおよびＣＤの情報の記録および再生を行う光ヘッド装置の例であり、図２を用いて説明する。光源として、ＤＶＤ用の波長λ_１（＝６６０ｎｍ）の光束Ｌ_１を出射する半導体レーザ１ＡとＣＤ用の波長λ_２（＝７８０ｎｍ）の光束Ｌ_２を出射する半導体レーザ１Ｂとの２種の光源を備えており、コリメートレンズ４と対物レンズ５との間に開口制限素子１０１を設置した。
【００３５】
また、ビームスプリッタ２として、光束Ｌ_１に対してはハーフミラーとなり、光束Ｌ_２に対しては全透過するビームスプリッタを用いた。ビームスプリッタ３として、光束Ｌ_１に対しては全透過し、光束Ｌ_２に対してはハーフミラーとなるビームスプリッタを用いた。また、ＣＤ７への情報の記録時に半導体レーザ１Ｂへの戻り光を防ぎ、半導体レーザ１Ｂのレーザ光の発振を安定させるため位相板９である４分の１波長板も搭載した。
【００３６】
開口制限素子１０１（図１）を以下の方法で作製した。まず、透明基板１１と透明基板１２それぞれの一方の面に、光束Ｌ_１および光束Ｌ_２に対し反射率が０．５％以下である反射防止膜１５、１６を真空蒸着法にて形成した。さらに、透明基板１１の他方の面には、表１に示す構成の波長選択性の誘電体多層膜１３を、イオンアシスト蒸着法で形成した。ここで、誘電体多層膜１３の平均屈折率は１．５７５であった。
【００３７】
【表１】

【００３８】
次に、フォトリソグラフィ法を用いて図１の（ｂ）に示す周辺領域Ｂにフォトレジストが残るようにパターニングして、中央領域Ａの部分をドライエッチング法にて食刻した後、周辺領域Ｂに残ったフォトレジストを除去した。次に、誘電体多層膜１３の中央領域Ａの部分に、屈折率が１．５７５の接着剤１４が充填されるように、透明基板１１と透明基板１２とを接着し固定した。このとき、接着剤１４の屈折率と波長選択性の誘電体多層膜１３の平均屈折率が等しくなるので、開口制限素子１０１を透過する光束Ｌ_１のうち、中央領域を透過する部分と周辺領域を透過する部分とで位相が揃った。
【００３９】
上記のように、層数が２０層前後の誘電体多層膜の成膜工程が１つ、ドライエッチング工程が１つ、接着工程が１つの単純な工程で作製した開口制限素子１０１を搭載した光ヘッド装置において、ＤＶＤおよびＣＤの厚さの異なる光ディスクに対し、１つの光学系で高い集光性能を実現でき、良好な記録再生特性を示した。
【００４０】
【発明の効果】
上記において説明したように本発明によれば、層数が２０層前後の誘電体多層膜の成膜工程が１つ、ドライエッチング工程が１つ、接着工程が１つと、製造工程数が少なく容易に開口制限素子を作製でき、かつ生産歩留まりのよい開口制限素子を得ることができる。さらに、この開口制限素子を光ヘッド装置に搭載することで、異なる２種の波長に応じて光束の開口の実効的な制限を行うことができ、厚さの異なるＤＶＤとＣＤとに対して、それぞれの光束を集光でき、情報の記録および再生を良好に行うことができる。
【図面の簡単な説明】
【図１】本発明の第１実施態様の開口制限素子の構成を示す図で、（ａ）断面図、（ｂ）平面図。
【図２】本発明の光ヘッド装置の構成例を示す概念的断面図。
【図３】本発明の第１実施態様の開口制限素子の製造方法を示す概念図。
【図４】本発明の第２実施態様の開口制限素子の構成を示す断面図。
【図５】本発明の第２実施態様の開口制限素子の製造方法を示す概念図。
【図６】本発明の第３実施態様の開口制限素子の構成を示す断面図。
【図７】本発明の第４実施態様の開口制限素子の構成を示す断面図。
【図８】本発明の第５実施態様の開口制限素子を搭載した光ヘッド装置の概念的断面図。
【図９】本発明の第５実施態様の開口制限素子の構成を示す断面図。
【図１０】本発明の第６実施態様の開口制限素子の構成を示す断面図。
【図１１】従来の開口制限素子の構成を示す断面図。
【符号の説明】
１０１、１０２、１０３、２０１、２０２、２０３、５０１：開口制限素子
１Ａ、１Ｂ：半導体レーザ
２、３：ビームスプリッタ
４：コリメートレンズ
５：対物レンズ
６：ＤＶＤ
７：ＣＤ
８、８Ａ、８Ｂ：光検出器
９：位相板
１０：偏光依存性のないホログラム素子
４１Ａ：ユニット
４１Ｂ：ホログラムユニット
１１、１２、１７、５１：透明基板
１３、１８：波長選択性の誘電体多層膜
１４、１９：接着剤
１５、１６、５４：反射防止膜
２１：偏光ホログラム格子
２２：フォトレジスト
２３：光の状態を制御する光学素子
５２：位相差調整用の誘電体多層膜
５３：波長選択性の誘電体多層膜
Ａ、Ｃ：中央領域
Ｂ、Ｄ：周辺領域[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an aperture limiting element and an optical head device, and more particularly to an optical head device that records and reproduces information on an optical recording medium.
[0002]
[Prior art]
CD / DVD compatible optical head devices that record and play back CDs or DVDs, which are optical discs of different standards, using the same optical head device have been commercialized. Since optical discs for CD and DVD have different standards such as the wavelength band of light used for recording and reproduction, the thickness of the optical disc, and the recording density, optical recording is performed for recording and reproducing a CD and for recording and reproducing a DVD. It is necessary to change the numerical aperture of the system.
[0003]
[Problems to be solved by the invention]
In order to perform recording and reproduction of a CD using an objective lens designed for DVD with a wavelength of 650 nm and having a numerical aperture of about 0.6, an aperture limiting element 501 shown in FIG. On the other hand, a method of switching the numerical aperture has been proposed. The aperture limiting element 501 has a central region C in which a dielectric multilayer film 52 for phase difference adjustment is formed on a transparent substrate 51 and a peripheral region D in which a wavelength selective dielectric multilayer film 53 is formed. . Reference numeral 54 denotes an antireflection film.
[0004]
The wavelength-selective dielectric multilayer film 53 has a function of transmitting a DVD light beam but not transmitting a CD light beam. On the other hand, the dielectric multilayer film 52 for adjusting the phase difference transmits both the DVD light flux and the CD light flux and has a function of aligning the phases of the DVD light fluxes in the central area C and the peripheral area D. Yes.
[0005]
However, in the manufacturing process for forming the central region C and the peripheral region D, the aperture limiting element 501 forms the dielectric multilayer film of about 20 layers twice, forms the lift-off mask twice, and lifts off twice. A complicated process such as repeated times is required. Due to this complicated process, the improvement of the yield is hindered, and an aperture limiting element manufactured by a simplified process has been desired.
An object of the present invention is to provide an aperture limiting element that can be easily manufactured with a small number of manufacturing steps and has a high production yield, and an optical head device using the aperture limiting element.
[0006]
[Means for Solving the Problems]
The present invention, among the two light fluxes entering shine that different wavelengths lambda ₁ and the wavelength _{_{_{λ 2 (λ 1 <λ 2}}} ), a transparent area that transmits filter area and both of the light flux transmitted through only one of the light beams The aperture limiting element includes two transparent substrates, and is formed of a wavelength-selective dielectric multilayer film having a smooth surface in a peripheral region between the two transparent substrates. In the central region where the filter region is formed and surrounded by the peripheral region, a concave portion having a certain level difference with respect to the peripheral region is provided on the side of the surface of the one transparent substrate facing the other transparent substrate. The transparent area formed of the transparent medium is formed, and the film configuration of the dielectric multilayer film and the refractive index of the transparent medium are determined with respect to the filter area and the light flux that passes through the transparent area. The above Providing an aperture limiting element is characterized in that in data region and the transparent region is adjusted such that the phase of the light beam is aligned.
[0007]
Further, the total thickness of the dielectric multilayer film and the average refractive index with respect to light having the wavelength λ ₁ are d _f2 and n _f2 , the refractive index of the transparent medium is n _s2 , and the step of the concave portion is d _a , Provided is the above aperture limiting element that satisfies the relationship of n _f2 · d _f2 + n _a · d _a = n _s2 · (d _f2 + d _a ) where n _a is the refractive index of the transparent substrate with respect to light of wavelength λ ₁ To do. In addition, there is provided the above aperture limiting element in which a wave plate and / or a diffraction grating made of an organic thin film is further stacked between the transparent substrates in addition to the dielectric multilayer film and the transparent medium.
[0008]
Also, two light sources that respectively emit two light beams having different wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ), an objective lens that condenses the two light beams on an optical recording medium, and An optical head device including a light detector that detects two light beams reflected by the optical recording medium, wherein the aperture limiting element is disposed in a common optical path of the two light beams between the two light sources and the objective lens. An optical head device is provided. Furthermore, out of two incident light beams having different wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ), a filter region that transmits only one of the light beams and a transparent refractive index n _s2 that transmits both light beams. a method of manufacturing a aperture limiting element and a transparent region including a medium, the aperture limiting element is on the plane of the transparent substrate where the refractive index is n _a with respect to the wavelength lambda ₁ of light in total thickness d _f2 An optical multilayer film having an average refractive index n _f2 with respect to light having the wavelength λ ₁ is formed, a photoresist is laminated on the optical multilayer film, and the optical multilayer film is etched by photolithography and etching only in the central area to be a transparent area. And the transparent substrate is etched to _a level difference d _a satisfying the relationship of n _f2 · d _f2 + n _a · d _a = n _s2 · (d _f2 + d _a ), and the resist in the peripheral region serving as the filter region is removed, Center Provided is a method of manufacturing an aperture limiting element in which a region is filled with the transparent medium and bonded and fixed to the other transparent substrate so that the surfaces of the two transparent substrates that are not opposed to each other are smooth.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 shows a configuration example of an optical head device on which the aperture limiting element 101 of the present invention is mounted. The optical head device shown in FIG. 2 is an optical head device that records and reproduces information on the DVD 6 and the CD 7. The optical laser device emits a light beam L ₁ having a wavelength λ ₁ for DVD as a light source and a wavelength λ for CD. and a semiconductor laser 1B for emitting a _second light beam L _2.
[0010]
The light beams L ₁ and L ₂ emitted from the

semiconductor lasers

1A and 1B are reflected by the

beam splitters

2 and 3, respectively, pass through the collimator lens 4, pass through the aperture limiting element 101, and pass through the aperture limiting element 101, and then the DVD 6 or CD 7 by the objective lens 5. Focused on top. The light reflected by the DVD 6 or CD 7 travels in the opposite direction to the above traveling direction, passes through the

beam splitters

2 and 3, and finally converges on the photodetector 8. Reference numeral 9 denotes a phase plate, which prevents the return light having the same polarization direction as the light emitted from the semiconductor laser 1B from entering the semiconductor laser 1B when recording information on the CD 7 by using a quarter-wave plate. The oscillation characteristics of the semiconductor laser 1B can be stabilized.
[0011]
FIG. 1A is a sectional view and FIG. 1B is a plan view of an aperture limiting element 101 as a first embodiment of the present invention. In the aperture limiting element 101, a wavelength-selective dielectric multilayer film 13 having a thickness of d _f1 is formed in the peripheral region B on the lower surface of the transparent substrate 11, and the transparent substrate 11 is bonded by an adhesive 14 that is a transparent medium. The transparent substrate 12 is fixed. At this time, the central region A is filled with the adhesive 14 so that the phase of the light beam L1 transmitted through the aperture limiting element 101 is aligned between the portion transmitting the central region A and the portion transmitting the peripheral region B. In addition, the film configuration of the dielectric multilayer film 13 and the refractive index of the adhesive 14 are adjusted.
[0012]
That is, the average refractive index n _f1 of the wavelength-selective dielectric multilayer film 13 and the refractive index n _s1 of the adhesive 14 are made equal. Here, the average refractive index means a value obtained by dividing the sum of the products of the refractive index and the thickness of each layer constituting the multilayer film by the total thickness of the multilayer film (the total thickness of each layer).
[0013]
Further, as shown in FIG. 1B, the aperture limiting element 101 has an antireflection film 15 on one surface of the transparent substrate 11 and an antireflection film on the other surface of the transparent substrate 12 in order to prevent reflection on the element surface. Each of the membranes 16 is preferably applied. As the

transparent substrates

11 and 12, optically isotropic materials such as glass and quartz glass can be used.
[0014]
Wavelength selective as the dielectric multilayer film 13, 90% or more transmittance for light beams _{L 1} having a wavelength lambda ₁ for DVD, the transmittance of 20% or less with respect to the light beam _{L 2} having a wavelength lambda ₂ for CD A dielectric multilayer film having about 20 layers can be used. However, the transmittance of the light beam L ₂ for CD, to prevent condensing of unnecessary light on the information recording surface of the CD, preferably 10% or less. Thus, the wavelength selective dielectric multilayer film 13 by forming a film on the peripheral region B, the peripheral region B filter region, the central region A becomes transparent region, the aperture limiting element 101 of the light beam L ₂ for CD The diameter can be limited to the diameter of the central region A.
[0015]
As the

anti-reflection film

15 and 16, the light beam L ₂ for the light beam L ₁ and the CD for the DVD is transmitted together 98%, easily dielectric multilayer film 4 of about five layers can be manufactured can be used .
[0016]
In the optical head device of FIG. 2 equipped with aperture limiting element 101 of the present invention, the light beam L ₁ for the DVD, the aperture limiting element 101, without being restricted diameter, numerical aperture by using the objective lens 5 0. 6 is condensed on the DVD 6 as a luminous flux L ₁ . On the other hand, the diameter of the light beam L ₂ for CD is limited by the aperture limiting element 101 and is condensed on the CD 7 as the light beam L ₂ having a numerical aperture of about 0.45 to about 0.5 using the objective lens 5. . In other words, optical discs having different thicknesses such as DVD6 and CD7 can be focused on the optical disc with a numerical aperture meeting each standard by using the same objective lens, so that recording and reproduction can be realized stably.
[0017]
Next, a manufacturing method of the aperture limiting element 101 of the present invention will be described with reference to FIG. First, as shown in FIG. 3 (a), one surface on the anti-reflection film 15 of the transparent substrate 11, the thickness of the other surface of the dielectric multilayer film 13 of the wavelength selective d _f1, vacuum deposition A film is formed by a sputtering method or a sputtering method. After that, as shown in FIG. 3B, a photoresist 22 is laminated on the dielectric multilayer film 13 and patterned by using a photolithographic technique. As shown in FIG. The body multilayer film 13 is etched by a dry etching method.
[0018]
Further, after the remaining photoresist 22 is peeled off, the transparent substrate 12 on which the antireflection film 16 is formed is adhered by using an adhesive 14, and the opening limiting element as shown in FIG. 101 is produced.
[0019]
FIG. 4 shows a sectional view of an aperture limiting element 201 as a second embodiment of the present invention. The aperture limiting element 201 includes a peripheral region in which the wavelength-selective dielectric multilayer film 18 is formed on one surface of the transparent substrate 17 and a central region etched in a concave shape, and is an adhesive 19 that is a transparent medium. Thus, the transparent substrate 17 and the transparent substrate 12 are fixed. The difference from the aperture limiting element 101 of the first embodiment is that a part of the central region is also etched on one surface of the transparent substrate 17.
[0020]
At this time, the central region is filled with the adhesive 19, and the dielectric L is formed so that the phase of the light beam L1 transmitted through the aperture limiting element 201 is aligned between the portion transmitting the central region and the portion transmitting the peripheral region. The film configuration of the body multilayer film 18 and the refractive index of the adhesive 19 are adjusted. That is, the total thickness d _f2 of the dielectric multilayer 18, the average refractive index n _f2 , the refractive index n _s2 of the adhesive 19, the step (depth) d _{a of} the portion etched into the concave _{shape of} the transparent substrate 17, the refractive index. with _{_{_{_{n a, n f2 d f2 +}}}} n a d a = n s2 to satisfy following _{_(d} f2 + d _a).
[0021]
In order to prevent reflection on the element surface, the aperture limiting element 201 has an antireflection film 15 on the other surface of the transparent substrate 17 and an antireflection film 16 on one surface of the transparent substrate 12, respectively, as shown in FIG. It may be given. As the

transparent substrates

17 and 12, optically isotropic materials such as glass and quartz glass can be used.
[0022]
Wavelength selective as the dielectric multilayer film 18, 90% or more transmittance for light beams _{L 1} having a wavelength lambda ₁ for DVD, the transmittance of 20% or less with respect to the light beam _{L 2} having a wavelength lambda ₂ for CD A dielectric multilayer film having about 20 layers can be used. However, the transmittance of the light beam L ₂ for CD, to prevent condensing of unnecessary light on the information recording surface of the CD, preferably 10% or less. Therefore, by forming the wavelength selective dielectric multilayer film 18 in the peripheral region, the peripheral region B filter region, the central region is the transparent section, the aperture limiting element 201 the diameter of the light beam L ₂ for CD The diameter of the central region A can be limited.
[0023]
In the optical head device of FIG. 2, even if the aperture limiting element 201 of the second embodiment is used in place of the aperture limiting element 101 of the first embodiment, similarly to the optical discs having different thicknesses such as DVD6 and CD7. Recording and reproduction can be realized stably.
[0024]
The

aperture limiting elements

101 and 201 of the present invention can be manufactured by a simple process such as one film forming process of a dielectric multilayer film having about 20 layers in terms of structure, and one dry etching process or lift-off process, An aperture limiting element can be manufactured with high productivity and high yield.
[0025]
Next, the manufacturing method of the aperture limiting element 201 of the present invention will be described with reference to FIG. First, as shown in (a) of FIG. 5, one anti-reflection film 15 on the surface of the transparent substrate 17, the thickness of the other surface of the wavelength selectivity of d _f2 dielectric multilayer film 18, vacuum deposition A film is formed by a sputtering method or a sputtering method. After that, as shown in FIG. 5B, a photoresist 22 is laminated on the dielectric multilayer film 18 and patterned by using a photolithographic technique. As shown in FIG. The body multilayer film 18 and the transparent substrate 17 are etched by a dry etching method.
[0026]
Further, after the remaining photoresist 22 is peeled off, the transparent substrate 12 on which the antireflection film 16 is formed is adhered by using an adhesive 19 to produce the aperture limiting element 201 shown in FIG. Is done.
[0027]
FIG. 6 is a cross-sectional view of an aperture limiting element 102 as a third embodiment of the present invention. In the aperture limiting element 102, an optical element 23 such as a phase plate or a diffraction grating is inserted at the interface between the adhesive 14 and the transparent substrate 12 in the aperture limiting element 101 of the first embodiment shown in FIG. This is the configuration. 6 that are the same as those in FIG. 1 indicate the same elements. When the aperture limiting element 102 of the third embodiment is mounted on an optical head device, the number of elements constituting the optical head device can be reduced, and the optical head device can be reduced in size, which is preferable.
[0028]
FIG. 7 is a cross-sectional view of an aperture limiting element 202 as a fourth embodiment of the present invention. In the aperture limiting element 202, an optical element 23 such as a phase plate or a diffraction grating is inserted at the interface between the adhesive 19 and the transparent substrate 12 in the aperture limiting element 201 of the second embodiment shown in FIG. This is the configuration. 7 that are the same as those in FIG. 3 indicate the same elements. When the aperture limiting element 202 according to the fourth embodiment is mounted on an optical head device, the number of elements constituting the optical head device can be reduced, and the optical head device can be reduced in size, which is preferable.
[0029]
FIG. 8 is a conceptual cross-sectional view showing an optical head device on which the aperture limiting element 103 according to the fifth embodiment of the present invention is mounted, and is an optical head device that records and reproduces information on DVD6 and CD7. 41A is a unit in which the semiconductor laser 1A for DVD and the photodetector 8A are integrated, and 41B is an integrated unitary integration of the semiconductor laser 1B for CD and the photodetector 8B and the hologram element 10 having no polarization dependency. It is a hologram unit. When a unit is used for the optical head device, the optical head device can be miniaturized. Note that the same reference numerals in FIG. 10 as those in FIG. 1 that are not described indicate the same elements as in FIG.
[0030]
For example, as shown in FIG. 9, the aperture limiting element 103 has a phase plate 20 and a polarization hologram grating 21 inserted at the interface between the adhesive 14 and the transparent substrate 12 in the aperture limiting element 101 of the first embodiment of FIG. (Fifth embodiment). The same reference numerals in FIG. 9 as those in FIG. 1 denote the same elements. Further, instead of the aperture limiting element 103, an aperture limiting element 203 shown in FIG. 10 may be used (sixth embodiment). The aperture limiting element 203 has a configuration in which the phase plate 20 and the polarization hologram grating 21 are inserted at the interface between the adhesive 19 and the transparent substrate 12 in the aperture limiting element 201 of the second embodiment of FIG. 10 that are the same as those in FIG. 4 indicate the same elements.
[0031]
The retardation value generated by the phase plate 20 in the aperture limit element 103 shown in FIG. 9, 5-quarter wavelength to the light beam L ₁ for DVD, to further the light beam L ₂ for CD so that the wave select. By this selection, the light beam L ₁ is transmitted straight without being diffracted by the aperture restricting element 103 in the forward path due to the function of the polarization hologram grating 21, but is diffracted by the aperture restricting element 103 in the return path after the DVD 6 reflection, and is detected The light is condensed on the container 8A (see FIG. 8). On the other hand, the light beam L ₂ can pass through the aperture limiting element 103 without being diffracted by the polarization hologram grating 21 in both the forward path and the return path.
[0032]
As the polarization hologram grating 21, for example, a holographic grating having a concavo-convex shape is processed into a polymer liquid crystal using photolithography and etching techniques, and then equal to the ordinary light refractive index or the extraordinary light refractive index of the polymer liquid crystal. An optically isotropic material filled in, for example, a concave portion of a hologram grating can be used.
[0033]
In the optical head device (FIG. 8) equipped with the aperture limiting element 103 according to the third embodiment of the present invention shown in FIG. 9, the aperture limiting element 101 shown in FIG. Similar to the optical head device, recording and reproduction of CDs and DVDs can be stably realized using a single objective lens. Further, the aperture limiting element 103 has a polarization hologram grating 21 and a phase plate 20 laminated, thereby providing an optical element with an aperture limiting function, a beam splitter function, and a polarization control function. Compared with the head device, a compact optical head device can be realized.
[0034]
【Example】
The present embodiment is an example of an optical head device for recording and reproducing information on DVD and CD, and will be described with reference to FIG. As a light source, two light sources of the semiconductor laser 1B for emitting a light beam _{L 2} of the semiconductor laser 1A and wavelength lambda ₂ for CD which emits a light beam _{L 1} having a wavelength lambda ₁ for DVD (= 660nm) (= 780nm ) The aperture limiting element 101 is installed between the collimating lens 4 and the objective lens 5.
[0035]
Further, as the beam splitter 2, it becomes a half mirror for the light beam L _1, with a beam splitter to totally transmitted for the light beam L _2. As the beam splitter 3, and the total transmittance for the light beam L _1, using a beam splitter as a half mirror against the light flux L _2. In addition, a quarter-wave plate as a phase plate 9 is also mounted to prevent return light to the semiconductor laser 1B when recording information on the CD 7 and to stabilize the oscillation of the laser light of the semiconductor laser 1B.
[0036]
The aperture limiting element 101 (FIG. 1) was produced by the following method. First, on one surface of the transparent substrate 11 and the transparent substrate 12, respectively, the reflectance to the light beam L ₁ and the light beam L ₂ is an

antireflection film

15 and 16 is 0.5% or less was formed by a vacuum deposition method. Further, a wavelength-selective dielectric multilayer film 13 having the configuration shown in Table 1 was formed on the other surface of the transparent substrate 11 by ion-assisted deposition. Here, the average refractive index of the dielectric multilayer film 13 was 1.575.
[0037]
[Table 1]

[0038]
Next, patterning is performed so that the photoresist remains in the peripheral region B shown in FIG. 1B using a photolithography method, and the portion of the central region A is etched by the dry etching method, and then the peripheral region B is etched. The remaining photoresist was removed. Next, the transparent substrate 11 and the transparent substrate 12 were bonded and fixed so that the adhesive 14 having a refractive index of 1.575 was filled in the central region A of the dielectric multilayer film 13. At this time, since the refractive index and the wavelength selectivity of the average refractive index of the dielectric multilayer film 13 of the adhesive 14 is equal among the light beams L ₁ passing through the aperture limit element 101, partial peripheral region transmitting a central region The phase is aligned with the part that passes through.
[0039]
As described above, a light mounted with an aperture limiting element 101 produced by a simple process of forming a dielectric multilayer film having about 20 layers, one dry etching process, and one bonding process. In the head device, high optical focusing performance can be realized with one optical system for optical discs having different thicknesses of DVD and CD, and good recording / reproducing characteristics were exhibited.
[0040]
【The invention's effect】
As described above, according to the present invention, the number of manufacturing steps is small, that is, one dielectric multilayer film having about 20 layers, one dry etching step, and one bonding step. Thus, it is possible to produce an aperture limiting element having a good production yield. Furthermore, by mounting this aperture limiting element on the optical head device, it is possible to effectively limit the aperture of the light beam according to two different wavelengths, and for DVDs and CDs having different thicknesses, Each light beam can be condensed, and information can be recorded and reproduced satisfactorily.
[Brief description of the drawings]
1A and 1B are diagrams showing a configuration of an aperture limiting element according to a first embodiment of the present invention, in which FIG. 1A is a cross-sectional view, and FIG.
FIG. 2 is a conceptual cross-sectional view showing a configuration example of an optical head device of the present invention.
FIG. 3 is a conceptual diagram showing a manufacturing method of the aperture limiting element according to the first embodiment of the present invention.
FIG. 4 is a cross-sectional view showing a configuration of an aperture limiting element according to a second embodiment of the present invention.
FIG. 5 is a conceptual diagram showing a manufacturing method of an aperture limiting element according to a second embodiment of the present invention.
FIG. 6 is a cross-sectional view showing a configuration of an aperture limiting element according to a third embodiment of the present invention.
FIG. 7 is a cross-sectional view showing a configuration of an aperture limiting element according to a fourth embodiment of the present invention.
FIG. 8 is a conceptual cross-sectional view of an optical head device equipped with an aperture limiting element according to a fifth embodiment of the present invention.
FIG. 9 is a sectional view showing a configuration of an aperture limiting element according to a fifth embodiment of the present invention.
FIG. 10 is a sectional view showing a configuration of an aperture limiting element according to a sixth embodiment of the present invention.
FIG. 11 is a cross-sectional view showing a configuration of a conventional aperture limiting element.
[Explanation of symbols]
101, 102, 103, 201, 202, 203, 501:

Aperture limiting element

1A, 1B: Semiconductor laser 2, 3: Beam splitter 4: Collimator lens 5: Objective lens 6: DVD
7: CD
8, 8A, 8B: Photodetector 9: Phase plate 10: Hologram element 41A without polarization dependence 41A: Unit 41B:

Hologram unit

11, 12, 17, 51: Transparent substrate 13, 18: Wavelength selective dielectric multilayer Films 14, 19:

Adhesives

15, 16, 54: Antireflection film 21: Polarization hologram grating 22: Photoresist 23: Optical element 52 for controlling the state of light 52: Dielectric multilayer film 53 for phase difference adjustment: Wavelength selection Dielectric multilayer films A, C: central region B, D: peripheral region

Claims

An aperture limiting element having a filter region that transmits only one of the two light beams having different wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ) and a transparent region that transmits both light beams. Because
The aperture limiting element includes two transparent substrates, and the filter region made of a wavelength-selective dielectric multilayer film having a smooth surface is formed in a peripheral region between the two transparent substrates,
In the central region surrounded by the peripheral region, a concave portion having a certain step with respect to the peripheral region is formed on the side of the surface of the one transparent substrate facing the other transparent substrate, and a transparent medium The transparent region is formed of,
In addition, the film configuration of the dielectric multilayer film and the refractive index of the transparent medium are adjusted so that the phase of the light beam is aligned in the filter region and the transparent region with respect to the light beam transmitted through the filter region and the transparent region. An aperture limiting element, characterized in that

The total thickness of the dielectric multilayer film and the average refractive index with respect to light having the wavelength λ ₁ are d _f2 and n _f2 , the refractive index of the transparent medium is n _s2 , the step of the concave portion is d _a , and the wavelength λ _When the refractive index of the transparent substrate for light of ₁ is n _a ,
_{_{_{_{n f2 · d f2 + n a}}}} · d a = n s2 · (d f2 + d a) the aperture limiting element according to claim 1 which satisfies the relation.

The aperture limiting element according to claim 1, wherein a wave plate and / or a diffraction grating made of an organic thin film is further laminated between the transparent substrates in addition to the dielectric multilayer film and the transparent medium.

Two light sources that respectively emit two light beams having different wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ), an objective lens that condenses the two light beams on an optical recording medium, and light that is condensed. An optical head device comprising a photodetector for detecting two light beams reflected by a recording medium, wherein the two light beams are in a common optical path between the two light sources and the objective lens. An optical head device in which the aperture limiting element according to the item is disposed.

Of two incident light beams having different wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ), a filter region that transmits only one light beam and a transparent medium having a refractive index n _s2 that transmits both light beams are used. A manufacturing method of an aperture limiting element having a transparent region including,
The aperture limiting element, the optical multilayer film is an average refractive index n _f2 with respect to the wavelength lambda ₁ of light in total thickness d _f2 on the plane of the transparent substrate where the refractive index is n _a with respect to the wavelength lambda ₁ of the light Forming and laminating a photoresist on the optical multilayer film, and etching the optical multilayer film by photolithography and etching only in the central area to be a transparent area and forming a transparent substrate with n _f2 · d _f2 + n _a · d _a = n _s2 -Etching to _a level difference d _a that satisfies the relationship (d _f2 + d _a ),
Remove the resist in the peripheral area that becomes the filter area,
A manufacturing method of an aperture limiting element in which the transparent medium is filled in the central region and bonded and fixed to the other transparent substrate, and the surfaces of the two transparent substrates that are not opposed to each other are smooth.